There are a number of processes for which ammonia is useful in a heated gas stream. In the case of air pollution control, examples are flue gas conditioning wherein a small amount of ammonia is injected and SCR system which depend on ammonia in relatively large amounts. Wherever ammonia is required in a hot gas stream, it would be desirable to avoid the danger and expense of dealing with ammonia per se.
SCR has been proven to be highly effective at NOx reduction, and SCR units can generally be scaled to the size required. However, SCR units typically require the use of ammonia as a reducing reagent, and it is a common problem that ammonia is difficult and dangerous to store, especially in populated areas. Thus, the use of urea and ammonia generators such as described in U.S. Pat. No. 7,090,810 to Sun, et al., and U.S. Pat. No. 6,077,491 to Cooper, et al., are often effective, but the ability to fully gasify the urea on an as-needed basis can cause problems if not done correctly.
When urea for such a gasification chamber or other like chemical for other commercial units is introduced, effective operation without fouling of equipment requires uniform distribution and rapid pyrolysis and/or hydrolysis. A proper velocity distribution of hot air before and after introduction of urea is critical for the operation of such a gasification chamber. While the concept of a perforated plate has been suggested to provide uniform flow prior to urea injection to provide a desirable gas pattern for urea distribution, in practice these devices can cause improper reagent back flow or recirculation which can result in solid urea encrusting on the plate, chamber walls or near the nozzle, causing fouling and related problems. It would be desirable to avoid fouling, especially on the nozzle.
There is a present need for a process, apparatus and system for efficient utilization of urea for purposes such as selective catalytic reduction (SCR) of NOx, and more particularly for gasification apparatus, methods and systems that enable feeding urea to a chamber designed to efficiently and completely gasify (by pyrolysis and/or hydrolyzation) the urea to feed a SCR unit.
There is a particular need for such a system which converts urea to ammonia, yet maintains the ability to fully control ammonia generation without equipment fouling or excessive reagent usage or loss of pollution control effectiveness.